This invention relates to centrifugal blowers and fans which have a rearwardly curved impeller.
Centrifugal blowers and fans generally include an impeller that rotates in a predetermined direction in a housing, and may be driven by an electric motor. The impeller has curved blades which draw air in axially, along the impeller's axis of rotation, and discharge air radially outwardly. Such blowers are used in a variety of applications, and blower design must account for many factors including the design points for pressure difference and airflow volume, motor power and speed, space constraints, inlet and outlet configuration, noise, and manufacturing tolerances.
One important design feature in a centrifugal fan is the angle of the blade tip relative to a tangent to the tip. This angle (.theta. in FIG. 2) is called the "blade exit angle". If the blade exit angle is greater than 90.degree., the impeller is said to have forwardly curved blades; if the blade exit angle is less than 90.degree., the impeller is said to have rearwardly curved blades.
In general, forwardly curved blades provide relatively large total pressure differences in comparison to rearwardly curved blades, all other things being equal. However, more of the pressure differential generated by forwardly curved blades is dynamic, rather than static--static pressure being the pressure differential across the impeller, and dynamic pressure being the pressure differential inherent in the kinetic energy of moving fluid. In order to recapture the dynamic pressure, forwardly curved blowers generally include a volute, which adds to the space required by the blower.
FIGS. 1A and 1B are highly diagrammatic representations of one form of conventional forwardly curved centrifugal blower with a volute. In the blower of FIGS. 1A and 1B, a significant percentage of the total pressure differential is dynamic, and a volute C is included to recover that pressure. The airflow at outlet D is uniform, but the overall space required for the blower is high, in that total width F is high relative to inlet diameter G and outlet dimension H.
Often, rearwardly curved blowers can be used without volute, since most of the total pressure difference is static pressure. FIGS. 2A and 2B are highly diagrammatic representatives of one form of conventional rearwardly curved centrifugal blower. In FIGS. 2A and 2B, the backwardly curved blades J of blower provide a relatively high percentage of pressure differential as static pressure rise, and the volute is not so likely to be required. However, airflow at the outlet is not uniform, and the ratio of the inlet dimension M to outlet dimension L is low, which can increase inlet losses.
In some applications, uniformity of discharge velocity over a relatively large area is desirable, for example, when the air is discharged from one side of the blower to a heat exchanger or to a cage of computer cards. In the former case, the pressure drop through the heat exchanger increases if the flow is non-uniform. In the latter case, air flow volume must be designed to accommodate the temperature of the most critical component, and a non-uniform airflow increases the design point for the volume of airflow, all other things being equal.
The above-described rearwardly-curved blade design tends to concentrate airflow along the open side of the impeller, as indicated by the arrows in FIG. 2A.
Specific centrifugal blowers described in prior patents are discussed below.
Koger et al., U.S. Pat No. 4,526,506 and DE 2,210,271disclose rearwardly curved centrifugal blowers with a volute.
GB No. 2,080,879 discloses a rearwardly curved centrifugal blower with stator vanes to convert radial flow to axial flow.
Samson, U.S. Pat. No. 3,829,250 discloses a backward-curved blower wheel in a housing. The blower wheel induces a flow of air in a generally radially outward direction, and the housing extends continuously around the circumference of the wheel to provide a plenum and to direct airflow axially. Airflow discharge is from the face opposite the inlet.
Zochfeld, U.S. Pat. No. 3,597,117 and GB No. 2,063,365disclose forwardly curved centrifugal blowers with a volute.
Calabro, U.S. Pat. No. 3,967,874 discloses a blower 16 positioned in a plenum chamber 14. The blade configuration and blower design are not apparent, but opening 46 in the bottom of the plenum chamber is in communication with the blower outlet.
GB No. 2,166,494 discloses a centrifugal impeller in a rotationally symmetrical cone-shaped housing, with guide vanes to produce an axial discharge.
GB No. 1,483,455 and GB No. 1,473,919 disclose centrifugal blowers with a volute.
GB No. 1,426,503 discloses a centrifugal blower with dual openings
Shikatani et al., U.S. Pat. No. 4,269,571 disclose a centripetal blower, which draws air in axial entrance 26 and out of the top periphery of disc 22 and axial exit 27 (3:26-36).
Canadian No. 1,157,902 discloses a rearwardly curved centrifugal blower with a curved sheet-metal guide.
Edmainer et al., U.S. Pat. No. 4,086,886 discloses a radial blower for a radiator block. A bladeless annular space 18 is provided between the blower rotor 12 and the radiator block 11.
Pottebaum, U.S. Pat. No. 4,662,830 discloses a centrifugal fan which draws air across a circuit board.